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Old 03-15-2012, 04:33 PM
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Vacuum needle bouncing 350 sbc

Hello all. I am in the middle of timing my engine. I have had so much trouble with getting things where I want it. Here's where I'm at... I have unhooked the vacuum advance all together and plugged the port on the carb. I have set the timing to 25 degrees BTDC, I have a Mr. Gasket recurve weight and spring kit for the HEI and I am using the medium weight springs. At 25 degree initial I end up with 34 degrees total and it's pretty spunky. Starts and stops fine but I notice with the vacuum gauge, unless I am idling at like 1200 rpm the needle is bouncing all over the place. At 1200 rpm the the needle only bounces slightly. If I rev up the engine the needle is steady but at idle it's not pretty. Oh btw... I have Crane Cam CHS - 274 H06 3/4R with hydraulic flat tappet liffters, 1.6:1 roller rockers. Its a 350 sbc punch .060" over. 10.2:1 static compression ratio. Edelbrock 600 electric choke carb. Can anyone please help point in a the right direction here? Thanks I have a video I will post after this.

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Old 03-15-2012, 04:38 PM
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Old 03-15-2012, 07:54 PM
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I have also noticed a slight miss when the needle bounces more than vibrates. I need to check the plugs and see what they look like. If it's worth anything the plugs are one heat range cooler than OE (95 K motor) since the compression is as high as it is.
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Old 03-15-2012, 07:55 PM
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I'm just about on the verge of buying an MSD E-curve dizzy and dropping the car off at a dyno shop for tuning.
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Old 03-15-2012, 10:32 PM
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Why are you setting it at 25 degrees static timing?? IMO you need to drop it back to 14-16 and recurve your distributor.
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Old 03-16-2012, 03:16 AM
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Quote:
Originally Posted by Caballerokid
I'm just about on the verge of buying an MSD E-curve dizzy and dropping the car off at a dyno shop for tuning.
Youll be sorry if you do!
the Ecurve is a POS!
I had mine RMAed twice then gave up and bought a mallory unilite distributor.
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Old 03-16-2012, 03:23 AM
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Okay Kid, Here's what I would do. First off, what shape is the mechanical advance in? Does it move smoothly? or does it have spots where it seems sticky? If it's smooth without slack we can move on. I would remove one of the medium springs and add a light spring. Next is I would slow the timing back down to 12 degrees before top dead center, then I would reconnect vacuum advance to a manifold vacuum source, one that pulls vacuum full time, including idle. Bring the idle back down to normal, around 750 or so. Now recheck timing, it should be 18 - 24 degrees before top dead center.
Now, disconnect vac advance and plug it, now check the full mechanical advance, you should be shooting for 36 degrees total, all in by 3000 RPM.
If you feel any stickyness in the mechanical advance find out where and what is causing it and take care of it. Any time I'm doing a build I closely inspect the mechanical advance and most times I'll rebuild the whole distributor and sand and polish the entire advance curve.
There could be several things that cause the vacuum gauge to bounce, especially on a engine with higher mileage. I'll also ask, what shape is the tune up in? What type of plug wires are you using? Once you get the things in order I mentioned then recheck your vacuum readings. If the needle still has a bounce then we'll dive further into where the low RPM miss is coming from and what's causing the needle to bounce.
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Old 03-16-2012, 12:11 PM
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Let me just start off by saying that the motor is not aged or aging. It is a brand new fresh rebuild. Everything brand new except the rods and crank. 68Nova, I bumped the timing up cuz it never really seemed to like the timing lower. I originally had the initial timing set at 12 BTDC with the old mechanical weights and springs. With the vacuum advance disconnected I had total mechanical advance of 22 degrees. At idle I would then connect the vacuum advance to ported vacuum and the timing would jump to 42 degrees advanced!! This is why I have started playing with multiple timing setups and even eliminating the vacuum advance and bumping the initial up so high that way I would still get a total advance around 34 degrees. I can get some pictures of the original weights and springs and what I have now. The old weight bushings were shot so I replaced the bushings and installed the new weights. They do move/pivot freely, but I have noticed that I have to modify the springs and make them tighter to get the weights to retract completely. There doesn't seem to be a bind anywhere. I don't know at this point. It is an older distributor but has never given me any trouble so I'm thinking there is just something with the setup since the engine is far from stock. I need to look into the dizzy more which is why I am strongly considering replacing it. I have a brand new MSD cap and rotor, OE replacement coil and ignition module. The wire are brand new 8.5mm MSD wires in full looms to secure them.

Last edited by Caballerokid; 03-16-2012 at 12:16 PM.
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Old 03-16-2012, 12:47 PM
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I will try more of the things you all have mentioned this weekend as well as get more pictures. I'm just getting really frustrated because I'm worried about damaging the engine. I just want to be able to drive and enjoy it.
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Old 03-16-2012, 03:40 PM
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That's why you time the base and total with vacuum advance disconnected and plugged then reconnect it afterwards to a manifold source, when it's done this way it's common for it to see up to 52 degrees total timing at cruise RPM, that is at PART throttle, where vacuum is still present. When your at wide open throttle, there IS NO vacuum, and the vac advance will not operate or have any effect. Why do we add vacuum advance from a manifold source at idle and cruise? Because at idle and cruise the fuel mixtures are lean, and lean mixtures burn SLOWER than do richer ones and thus more advance is needed. You don't have to believe me, you don't even have to try it. All up to you. I've been over this very same thing with many on this board who also wouldn't try it as they believed I couldn't possibly be right. Maybe if they put down the comic books they read about setting the spark timing they would learn and understand what I'm saying here. I've timed more engines this way than many of them will build in a life time and know it works quite well on street engines.
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Old 03-18-2012, 08:08 PM
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I started working with the mechanical advance making extra sure everything was working correctly. When I was done I ended up with no spark, I had 12V to the coil (in cap) but no spark output. It was all working fine the night before. In cleaning everything up under the cap I think I may have created some carbon tracking. I am fed up with this dist nickle and dimeing me. I ordered a MSD e-curve dist , blaster SS coil and a supporting excel file to aid in setting it all up. I am currently waiting on the new parts. I have no doubt that you are right about the timing doublevision, I was just worried about 42 degrees advance at idle as I am currently running wide open headers and 110 leaded fuel. It will not be so easy to hear pinging due to those things. I have more money than I would like to know wrapped up in this engine and do not want to ruin the engine over ignition timing. You know what I mean... I just want to do it right from the start and not have to worry about it later. Thank you for the input. I'm sure I will have some questions with the new distributor as MSDs instructions are quite confusing. I am reading up on it as much as I can before I get it in hand. Thanks again!
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Old 03-18-2012, 08:27 PM
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A few of the things I found while I was working on it this weekend. The plugs are R43T (should be one range cooler than OE) gapped at .045" they look a little whiteish but that's prlly the leaded 110. They don't look bad to me, is there anything wrong with the coloration on the spark plug? The plug wires are well secured. One thing I did notice is that when I tightened down the rotor the mechanical advance wouldn't operator smoothly??? If I loosened up the rotor it would free up the mechanical advance??? I made sure that the rotor itself wasn't causing the bind anywhere but I think when the screws are tightened it pulls the plate tight against the mechanical weight shaft therefore binding it up. I have a WD-40 pen that I used to make sure the weights and bushings were "lubed". I think once the WD-40 slung out it may have cause my carbon tracking because once I was done with all of this it never started again. Thus me ordering the e-curve! haha
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Old 03-18-2012, 08:37 PM
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Here's a couple more pictures for reference. Oh, and the engine is freshly built with like 5 miles on it, fwiw.
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Old 03-18-2012, 09:03 PM
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Here's the video of the mechanical advance mechanism. It shows the bind just near the home position. I think the thing is just shot... anyone confirm that?
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Old 03-18-2012, 09:40 PM
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I never let mine operate like that which is why I polish everything so it all works silky smooth. Also I tip my hat to you, you listened and learned.
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